Merge branch 'TMSS-662' into 'master'

TMSS-662: fix another aggregation issue when SU has no on_sky_duration

Closes TMSS-662

See merge request !1249

SAS/TMSS/backend/src/tmss/tmssapp/adapters/reports.py

@@ -709,7 +709,7 @@ class FailureReport():
         affected_tasks = models.TaskBlueprint.objects.filter(system_events__in=system_events)
         affected_units = (models.SchedulingUnitBlueprint.objects.filter(task_blueprints__in=affected_tasks))
 
-        for unit in affected_units.all():
+        for unit in affected_units.exclude(on_sky_start_time__isnull=True).all():
             start_bin = datetime(year=unit.on_sky_start_time.year, month=unit.on_sky_start_time.month, day=1)
             histogram[start_bin] = histogram.get(start_bin, 0) + (unit.on_sky_duration.total_seconds())